The quadrupole ion trap, sometimes referred to as an ion store or an ion trap detector, is a well-known device for performing mass spectroscopy. A ion trap comprises a ring electrode and two coaxial end cap electrodes defining an inner trapping volume. Each of the electrodes preferably has a hyperbolic surface, so that when appropriate AC and DC voltages (conventionally designated "V" and "U", respectively) are placed on the electrodes, a quadrupole trapping field is created. This may be simply done by applying a fixed frequency (conventionally designated "f") AC voltage between the ring electrode and the end caps. The use of an additional DC voltage is optional.
Typically, an ion trap is operated by introducing sample molecules into the ion trap where they are ionized. Depending on the operative trapping parameters, ions may be stably contained within the trap for relatively long periods of time. Under certain trapping conditions, a large range of masses may be simultaneously held within the trap. Various means are known for detecting ions that have been so trapped. One known method is to scan one or more of the trapping parameters so that ions become sequentially unstable and leave the trap where they may be detected using an electron multiplier or equivalent detector. Another method is to use a resonance ejection technique whereby ions of consecutive masses can be sequentially scanned out of the trap and detected.
The mathematics of the trapping field, although complex, are well developed. Ion trap users are generally familiar with the stability envelop diagram depicted in FIG. 1. For a trap of a given radius r.sub.0 and for given values of U, V and f, whether an ion of mass-to-charge ratio (m/e) will be trapped depends on the solution to the following two equations: ##EQU1##
Where .omega. is equal to 2.pi.f.
Solving these equations yields values of a and q for a given m/e. If, for a given ion, the point (a,q) is inside the stability envelop of FIG. 1, the ion will be trapped by the quadrupole field. If the point (a,q) falls outside the stability envelop, the ion will not be trapped and any such ions that are created within the trap will quickly depart. It follows that by changing the values of U, V or f one can control whether a particular mass ion is trapped in the quadrupole field. It should be noted that it is common in the field to use the terms mass and mass-to-charge ratio interchangeably. However, strictly speaking, it proper to use the term mass-to-charge ratio.
In the absence of a DC voltage, the equations set forth actually relate to stability in the direction of the z axis, i.e., the direction of the axis of the electrodes. Ions will become unstable in this direction before becoming unstable in the r direction, i.e., a direction radial to the axis. Thus, it is normal to limit consideration of stability to z direction instability. The differential in stability results in the fact that unstable ions will leave the trap in the z direction, i.e., axially.
In commercially available implementations of the ion trap, the DC voltage, U, is set at 0. As can be seen from the first of the above equations, when U=0, then a.sub.z =0 for all mass values. As can be seen from the second of the above equations the value of q.sub.z will be inversely proportional to the mass of the particle, i.e., the larger the value of the mass the lower the value of q.sub.z. Likewise, the higher the value of V the higher the value of q.sub.z. Turning to the FIG. 1 stability envelop, it can also be seen that for the case where U=0, and for a given value of V, all masses above a certain cut-off value will be trapped in the quadrupole field. Although all masses above a cut-off value are stable in such a trapping field, there are limits to the quantity of ions of a particular mass value that will be trapped due to space charge effects. As discussed below such quantity limitations are also a function of the magnitude of V.
Several methods are known for ionizing sample molecules within the ion trap. Perhaps the most common method is to expose the sample to an electron beam. The impact of electrons with the sample molecules cause them to become ionized. This method is commonly referred to as electron impact ionization or "EI".
Another commonly used method of ionizing sample with an ion trap is chemical ionization or "CI". Chemical ionization involves the use of a reagent gas which is ionized, usually by EI within the trap, and allowed to react with sample molecules to form sample ions. Commonly used reagent gases include methane, isobutane, and ammonia. Chemical ionization is considered to be a "softer" ionization technique. With many samples CI produces fewer ion fragments than the EI technique, thereby simplifying mass analysis. Chemical ionization is a well known technique that is routinely used not only with quadrupole ion traps, but also with most other conventional types of mass spectrometers such as quadrupole mass filters, etc.
Other, more specialized, methods of ionization are also in use in mass spectroscopy. For example, photoionization is a well known technique that, similar to electron impact ionization, will affect all molecules contained in the trap.
Most ion trap mass spectrometer systems in use today include a gas chromatograph ("GC") as a sample separation and introduction device. When using a GC for this purpose, sample which elutes from the GC continuously flows into the mass spectrometer, which is set up to perform periodic mass analyses. Such analyses may, typically, be performed at a frequency of about one scan per second. This frequency is acceptable since peaks typically elute from a modern high resolution GC over a period of several seconds to many tens of seconds. When performing CI experiments in such a system, a continuous flow of reagent gas is maintained. As a practical matter it is undesirable to interrupt the flow of sample gas from the GC to the ion trap. Likewise, when conducting both CI and EI experiments on a sample stream, it is undesirable to interrupt the flow of reagent gas to the ion trap.
When performing CI, it is necessary to ionize a reagent gas, which then chemically reacts with and ionizes the sample gas. As noted, electron impact ionization within the ion trap is the preferred method of ionizing the reagent gas. However, if sample is present in the ion trap when the electron beam is turned on to ionize the reagent gas, the sample will also be subject to EI. As noted above, where chromatography is used to separate a sample before it is introduced into the ion trap, it is impractical to interrupt the flow of sample gas. Therefore, there is not a practical way to ionize the reagent gas without also ionizing the sample. Thus, unless mitigating measures are taken, sample ions will be formed by both CI and EI, leading to potentially confused results.
The prior art solution to this problem is described in U.S. Pat. No. 4,686,367, entitled Method of Operating Quadrupole Ion Trap Chemical Ionization Mass Spectrometer, issued on Aug. 11, 1987, to Louris, et al. The method of the '367 patent seeks to minimize the effects of EI of the sample by minimizing the number of sample ions trapped by the ion trap while reagent gas is being ionized. The method that is taught for doing this is to apply a low value of V to the trap during the EI step so that the low mass reagent ions will be trapped, but the number of high mass ions will be small. In the words of the patent, "at sufficiently low RF values, [i.e., values of V] high molecular weight ions are not efficiently trapped. So, at low RF voltages only the low mass ions are stored." (Column 5, lines 33-36.)
As is explained above, when operating using the RF only method, which is preferred in the '367 patent and which is the method used in all known commercial embodiments of the ion trap, the trap inherently traps all masses above a cut-off mass which is set by the value of the RF trapping voltage. Thus, to trap low mass ions, whether they be reagent ions or sample ions, it is necessary to set V at a sufficiently low value. When V is set low enough the trap inherently has a poor efficiency in trapping high mass ions due to space charge effects. A theoretical way of looking at this is that the volume of the interior of the ion trap which stores ions of a particular mass is proportional to the value of V and is inversely proportionally to the mass. Thus, for any given V a smaller volume of the ion trap is available to store high mass ions than low mass ones. When the volume is quite small the number of ions that can be stored is reduced due to space charge effects.
It should be noted that setting a low value of V does not cause all high mass ions to leave the trap; such ions continue to have values of a and q that map into the stability envelop. All that can be done following the technique of the '367 patent is to reduce the number of high mass ions in the trap during the EI step. In this respect, the statement in the patent that "at low RF voltages only the low mass ions are stored" appears to be incorrect. As described below, experimental results show the presence of detectable quantities of high mass ions created by EI in experiments conducted using the method of the '367 patent. Moreover, the number of high mass ions that remain trapped will depend on the mass, so that a substantial number of sample ions close, yet higher, in mass than the reagent ions, will be trapped.
Some reagent molecules form a variety of ions having different masses. Ionization at RF voltages substantially below that necessary to trap the lowest mass reagent ion, which is necessary to remove most of the high mass sample ions, will reduce the number of reagent ions that are trapped, as well as the high mass sample ions. This effect is related to mass so that the higher mass reagent ions will be disproportionately lost from the trap.
A related problem exists when conducting both EI and CI experiments on a single sample stream in an ion trap. As noted above, for practical reasons it is undesirable to stop the flow of reagent gas to the trap. However, if reagent gas is present when an EI experiment is run, the reagent gas will be ionized creating reagent gas ions which may cause CI of the sample unless they are eliminated from the trap before reactions can occur. This problem does not exist when conducting only EI experiments on a sample stream since the reagent gas flow may simply be kept off during such experiments.
The method of the lowering the trapping voltage is not applicable, however, to solving this problem since it would not eliminate low mass reagent ions from the trap. One solution used to solve this problem, as taught in the '367 patent, is to raise the RF trapping voltage so as not to store the low mass reagent ions. However, this has the undesired effect of changing the trapping conditions from those which are normally used. For example, when the trapping voltage is set to store ions of mass 20 and above, the average ionizing energy of electrons entering the trap is 70 eV. Raising the trapping voltage to store only ions of mass 45 and above, so as to eliminate methane reagent ions at mass 43, would double the average electron energy. Such an increase would change the mass spectrum of many compounds and would reduce the trapping efficiency for the sample ions.
In a CI process it is desirable to optimize the number of product ions that undergo mass analysis. If there are too few product ions, the mass analysis will be noisy, and if there are too many product ions resolution and linearity will be lost. The formation of product ions is a function of the number of reagent ions present in the trap, the number of sample molecules in the trap, the reaction rate between the reagent ions and the sample ions, and the reaction time during which reagent ions are allowed to react with sample molecules. One can increase the number of reagent ions present in the trap by increasing the EI ionization time, i.e., keeping the electron beam on a longer time. Likewise, one can increase the number of sample ions formed in the trap by increasing the reaction time.
One prior art method of addressing this issue is set forth in U.S. Pat. No. 4,771,172, entitled Method Of Increasing The Dynamic Range And Sensitivity Of A Quadrupole Ion Trap Mass Spectrometer Operating In The Chemical Ionization Mode, issued on Sep. 13, 1988, to Weber-Grabau, et al. This patent covers a method of adjusting the parameters used in an ion trap in the CI mode so as to optimize the results. In order to optimize the parameters, the patent teaches the method of performing a CI "prescan," done in accordance with the method of the '367 patent, preceding each mass analysis. This prescan is a complete CI scan cycle in which the ionization and reaction times are fixed at values smaller than those that would be used in a normal analytical scan, and in which the product ions are scanned out of the trap faster than in a normal analytical scan. The resulting product ions that are ejected from the trap during the prescan are not mass resolved and the ion signal is only integrated to give a total product ion signal. During the prescan the total number of product ions in the trap are measured and the parameters, i.e., the ionization time and/or the reaction time for the subsequent mass analysis scan are adjusted.
Thus, the patent covers a two-step process consisting of first conducting a "prescan" of the contents of the ion trap to obtain a gross determination of the number of product ions in the trap, followed by a mass analysis scan of the type taught in the '367 patent, with the parameters of mass analysis scan being adjusted based on the data collected during the prescan. The disadvantage of the prior art method of extending the dynamic range by using a prescan to estimate the sample amounts in the trap is that it requires additional time to perform the prescan, and thus fewer analytical scans can be performed in the same time period. Not only does each of the prescans consume time, but each produces data which has no independent value apart from its use in adjusting the parameters for the mass analysis scan. However, adjustments in the mass analysis scan parameters are only required when conditions change. It is not necessary to make adjustments for each scan and, thus, in many instances the prescan step, in addition to consuming time, will not serve any useful purpose. Thus, there is a need for an improved method of adjusting the ion trap during chemical ionization experiments to operate within its dynamic range.
There is a demand to employ the ion trap mass spectrometer in conducting so-called MS.sup.n experiments. In MS.sup.n experiments, a single ion species is isolated in the trap and is dissociated into fragments. The fragments created directly from the sample species are known in the art as daughter ions, and the sample is referred to as the parent ion. The daughter ions may also be fragmented to create granddaughter ions, etc. The value of n refers to the number of ion generations that are formed; thus, in an MS.sup.2 or MS/MS experiment, only daughter ions are formed and analyzed.
A prior art method of conducting MS.sup.n experiments is described in U.S. Pat. No. 4,736,101, entitled Method Of Operating Ion Trap In MS/MS Mode, issued Apr. 5, 1988 to Syka, et al. After isolating an ion species of interest, the parent ions are resonantly excited by means of a single supplemental AC frequency which is tuned to the resonant frequency of the ions of interest. The amplitude of the supplemental frequency is set at a level which causes the ions to gain energy so that their oscillations within the trap are greater, but which is not large enough to cause the ions to be ejected from the trap. As the ions oscillate within the trap they collide with molecules of the damping gas in the trap and undergo collision induced dissociation thereby forming daughter ions. By applying resonant frequencies associated with the mass-to-charge ratios of the daughter ions, they can similarly be fragmented.
The difficulty with the method of the '101 patent is that the precise resonant frequency of the ions of interest cannot be determined a priori but must be determined a posteriori. The resonant frequency of an ion, also referred to as its secular frequency, varies with the ion mass-to-charge ratio, the number of ions in the trap, hardware variances and other parameters which cannot be precisely determined in a simple way. Thus, the precise resonant frequency of an ion species must be determined empirically. While empirical determination can be performed without great difficulty when a static sample is introduced into the trap, it is quite difficult to accomplish when a dynamic sample, such as the output of a GC, is used.
One prior art approach to overcoming the foregoing problem in determining the precise resonant frequency of a sample ion of interest is to use a broadband excitation centered around the calculated frequency. For example, such a broadband excitation may have a bandwidth of about 10 KHz. Another method is to conduct a frequency prescan, i.e., sweep the supplemental field across a frequency range in the area of interest and observe the resonant frequency empirically. However, neither of these solutions are particularly satisfactory.
Accordingly, it is an object of the present invention to provide an new method of eliminating sample ions created in an ion trap during ionization of a reagent gas, which is both simple and which has greater efficiency than methods known in the prior art, and without the need to change the RF trapping field between the ionization and reaction steps.
Another object of the present invention is to provide a method for conducting electron impact ionization experiments in an ion trap in the presence of a reagent gas, whereby reagent ions formed in the trap are eliminated from the trap before they are able to react with sample molecules.
Yet another object of the present invention relates to a method of optimizing the experimental parameters utilized in an ion trap in order to operate within dynamic range of the trap.
Still another object of the present invention is to provide a simple, yet highly effective, method for conducting MS.sup.n experiments in an ion trap that does not require the empirical determination of the resonant frequency of the sample species isolated in the trap.
Yet another object of the present invention is to provide an alternate method of scanning a trap to obtain a mass spectrum of its contents.